Not much I can add that DK hasn't touched on, but yeah, Chain Lightning was explicit about the difference between Wally and his cohort speedsters. Time travel for Flashes is involved because there is a speed component and a vibratory component. Both have to exist at extraordinarily high rates for time travel to take place. But yeah, in that story the other speedsters could not make the time jump without the aid of the cosmic treadmill and none were FTL but Wally.